Photovoltaic power generator systems are becoming more and more popular. For extracting the maximum available power from the photovoltaic modules, the system includes a maximum power point tracker (MPPT). Maximum power point trackers operate in such a manner that the output voltage and current of the modules are at an operating point that maximizes the power output from the module. There are multiple known tracking methods available which basically operate by changing the output voltage of the modules and calculate if the power from the modules increases or decreases. This calculation is carried out during the operation of the photovoltaic generator since the operating conditions can vary. The maximum power point may change for example due to clouds shadowing the modules or due to change of the relative position of the sun.
Photovoltaic modules are usually connected in series or parallel to increase the power rating of the power generator. When modules are connected in series, the voltage and, therefore, also the power ratings of the system increase. The series connection of modules can produce situations in which the series-connected modules receive irradiation unevenly. These situations may lead to a state in which the maximum power point tracker sets the operation point of the converter at the output of the series-connected modules to a power point that is not the maximum power point.
The electrical characteristic of a photovoltaic (PV) power generator under uniform irradiance conditions has only one local and, therefore, global maximum power point (MPP) at which the maximum available power can be extracted. This is illustrated in FIG. 1 (a) which shows the power of the generator as a function of voltage. If the generator is, for example, partially shaded (part of the series-connected PV modules are shaded), the electrical characteristic has two local MPPs. An example of the electrical characteristic under such a condition is shown in FIG. 1 (b). It can be seen in FIG. 1 (b) how the obtainable power has two local maximum points marked as MPP.
As mentioned above, the maximum power point tracker may set the converter of the solar photovoltaic power system to operate at a maximum power point that is not the global maximum power point. If the system includes a series connection of photovoltaic modules and the system may have multiple maximum power points, the MPP tracker may be operated in such a way that it scans through the operation points of the series connection to find out if there is an operation point from which more power could be extracted.
This scanning of the operation points is time-consuming since it has to be carried out regularly even if the operation of the system is in the only or global maximum power point. Further, the time the scanning of the operation point takes is lost from the operation at the maximum power point. During the repetitive sweeps, some energy is lost.